Black holes one billion times the sun’s mass or more lie at the heart of many galaxies, driving their spin and development. Common today, some 14 billion years after the big bang, such supermassive black holes were rare in the early universe—or at least they were supposed to be. Evidence of supermassive black holes existing when the universe was less than one billion years old has stumped scientists, because current theories of stellar evolution suggest that such giants should take much longer to grow. Now it seems this enigma could be solved by a mystery substance—dark matter.
The puzzle of early supermassive holes took shape in 2003, when the Sloan Digital Sky Survey detected roughly half a dozen of them. According to conventional thinking, the first regular stars were born when the universe was about 200 million years old, but given the state of the universe at the time, they could have formed black holes at most only about 100 times the sun’s mass. It would simply take too long to merge and make the billion-year-old, billion-solar-mass monsters seen by the Sloan survey.
Dark matter could solve the conundrum, say theoretical physicist Katherine Freese of the University of Michigan at Ann Arbor and her colleagues. Unseen but demonstrating its existence via gravity, the substance makes up at least 80 percent of the universe’s matter (and about one quarter of the entire universe). But scientists are unsure exactly what dark matter is made of. Among the leading hypothetical candidates are weakly interacting massive particles called neutralinos. They can annihilate one another when they meet, generating heat, gamma rays, neutrinos, and antimatter particles such as positrons and antiprotons.
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Freese and her co-workers calculate that when the universe was just 80 million to 100 million years old, as protostellar clouds of gas tried to cool and shrink, their gravity would have drawn in neutralinos that annihilated one another, unleashing energy that would have created the first stars. They dub these objects “dark stars,” fueled by dark matter rather than nuclear energy as in normal stars.
Their initial findings hinted that dark stars would have dwarfed regular stars. Because dark stars do not need the high densities seen in regular stars, which depend on atomic nuclei getting forced together for fusion, they would be much fluffier, with the largest ones reaching up to approximately 200,000 times the sun’s width. Scientists have also estimated that the cooler surface temperatures of dark stars would have allowed them to grow up to 1,000 times the mass of the sun, as compared with the roughly 150-solar-mass size limit of current stars.
Freese and her colleagues, who plan to submit their analysis to the Astrophysical Journal, figure that dark stars could have reached as much as 100,000 solar masses or more before they burned out their fuel and collapsed. They analyzed how frequently neutralinos would flow into dark stars and get captured by atoms, concluding that dark matter particles could have fueled the growth of dark stars for much longer than first thought.
After supermassive dark stars ran out of dark matter, they would have contracted, triggering nuclear fusion and continuing on as regular stars for roughly a million years. These stars would not have gone supernovae—“they are too big,” Freese says—instead they collapsed into black holes of the same mass. Several of them could have then merged into giants within a billion years of the big bang.
Supermassive dark stars would have been up to one billion times as bright as the sun and yet able to shine at our sun’s temperature with a yellow light. Freese hopes the James Webb Space Telescope, currently scheduled for launch in 2014, will see far enough to detect such fluffy giants. But no dark stars are likely to be forming today, because the density of dark matter now averages 1/8,000th of its dark star past, when the universe was far more compact.
Not everyone is convinced dark stars are real. Astrophysicist Brian O’Shea of Michigan State University contends the idea depends on too many assumptions regarding dark matter’s properties—for instance, if dark matter were instead made of another theoretical particle dubbed the invisible axion, which does not self-annihilate, dark stars could not form.
Still, astrophysicist Paul R. Shapiro of the University of Texas at Austin thinks dark stars “are reasonable consequences of a reasonable model for dark matter.” And if scientists do find dark stars, they could help elucidate more than just black holes—they would hint at what dark matter actually might be. “If dark stars do exist, they would be incredibly cool,” O’Shea notes.